A computer system may include a vision system to acquire video of a user, to determine the user's posture and/or gestures from the video, and to provide the posture and/or gestures as input to computer software. Providing input in this manner is especially attractive in video-game applications. The vision system may be configured to observe and decipher real-world postures and/or gestures corresponding to in-game actions, and thereby control the game. However, the task of determining a user's posture and/or gestures is not trivial; it requires a sophisticated combination of vision-system hardware and software. One of the challenges in this area is to accurately distinguish the user from a complex background.